The measurement of gas concentrations is gaining increasing importance in the field of consumer electronics (CE) as well as building safety technology and medical technology. This applies in particular for networked devices (IoTS). There is particular interest in the development of small economical sensors with a low power consumption, which can be integrated in large production numbers into existing devices. To this end, there are many different approaches. Mention may in this case be made particularly of resistive sensors, the conductivity variation of which as a function of the gas atmosphere is read out, and various types of field-effect transistors which detect the work function variation of the electrode material used. The former in principle presuppose a relatively good electrical conductivity of the gas-sensitive materials. The latter have the disadvantage of elaborate processing and relatively high power consumption, if heating is necessary. This is problematic in particular when the sensor elements are intended to be applied onto micro-heating plates, such as those which may for example be used in cell phones.
One important target gas for CE applications is carbon dioxide (CO2), for example in order to monitor the room air quality. Until now, infrared sensors which require a relatively large installation space have predominantly been used for CO2 measurement.
Solid electrolyte components, for example barium carbonate/gold electrodes, which operate at high temperatures, are also used.
It is known from Ostrick B., Mühlsteff J., Fleischer M., Meixner H., Doll T., Kohl C.-D., Absorbed water as key to room temperature gas-sensitive reactions in work function type gas sensors: the carbonate carbon dioxide system. Sens. Actuat. B-Chem. 1999; 57: 115-119, that barium carbonate already exhibits significant work function variations as a function of the CO2 partial pressure even at temperatures below 100° C. The readout of this effect with the aid of suspended-gate field-effect transistors is described in EP 0 947 829 A1. Such field-effect transistors have a dielectric layer which is arranged between two electrodes. One of these electrodes is configured as a composite electrode. Such a sensor structure is in principle also used for detecting other analytes. For example, a moisture sensor which is known from DE 2 221 656 A1 has a similar structure.
R. Plonka, R. Dittmann, N. A. Pertsev, E. Vasco, R. Waser, Impact of the top-electrode material on the permittivity of single-crystalline Ba0.7Sr0.3 TiO3 thin films, Appl. Phys. Lett. 86, 202909 (2005) describes the dependency of the impedance of a thin-film capacitor, which contains Ba0.7 Sr0.3 TiO3 as a dielectric, on the material of the electrodes used.